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Used properly, this feature can reduce considerably the security risks involved with allowing users to develop and run private CGI or SSI programs. However, if suEXEC is improperly configured, it can cause any number of problems and possibly create new holes in your computer's security. If you aren't familiar with managing setuid root programs and the security issues they present, we highly recommend that you not consider using suEXEC.
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... and this one from the same site:

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Second, it is assumed you are familiar with some basic concepts of your computer's security and its administration. This involves an understanding of setuid/setgid operations and the various effects they may have on your system and its level of security.
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Everything has changed since 2004. In fact, if you re-read that post from theplanet you'll see other folks join in with comments, including some experienced sysadmins who say "suexec was not the reason you were hacked" -- and it wasn't. You're probably new to server admin and security at least to some extent or you wouldn't ask what's changed in 3 years - the answer is, nearly everything!

Suexec as installed by WHM will work fine and will help your security a lot, don't hesitate. Those statements are old hangovers left in the doco, initially as a precaution against people blindly installing it and having problems due to something else stupid that they did then blaming the authors. I can't even think of a scenario where this might happen, and I haven't seen such a problem myself in 3 years on multiple servers.

The statement made about /tmp is completely ridiculous as in fact the reverse is true; it's in fact true that WITHOUT Suxec someone can create files in /tmp (and read other people's files, more importantly).